Atomicity of an element is a measure of the total number of atoms present in a molecule. For example, an oxygen molecule (written as O2) consists of 2 atoms of oxygen (O) and hence an atomicity of 2. Similarly, an ozone molecule (O3) consists of 3 atoms of oxygen and has an atomicity of 3.
There is a simple way to determine the atomicity of any element. It can be determined as a ratio of molecular mass and atomic mass. For example, using the case of oxygen molecule, the molecular mass is 32 (= 2x16), while the atomic mass is 16. If we divide the two, we get atomicity as 2 (=32/16).
Thus, atomicity of an element = molecular mass/atomic mass
We can use this relation to find the atomicity of any other elements as well.
Hope this helps.